Mobile station, base station, communications system, and communication method

ABSTRACT

A mobile station comprises a reception unit configured to receive a signal transmitted from a base station; a selection unit configured to select a communication area to perform communication from a plurality of communication areas based on traffic load in the plurality of communication areas where the reception unit can receive the signal; and a notification unit configured to notify the base station of the communication area selected. Another mobile station comprises a reception unit configured to receive a signal transmitted from a base station; a calculation unit configured to calculate a ratio of an average reception quality value over a short period to an average reception quality value over a long period; and a selection unit configured to select a communication area to perform communication from a plurality of communication areas where the reception unit can receive the signal, based on the ratio.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. P2002-61150, filed on Mar. 6,2002; the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mobile station, a base station, acommunication system, and a communication method.

2. Description of the Related Art

Conventionally, in a wideband-code division multiple access (W-CDMA)system, consecutive high quality communication is performed through softhandover. Soft handover refers to a mobile stationtransmitting/receiving a plurality of signals in a plurality of sectorsor cells having different spreading codes.

Meanwhile, in the case of soft handover in a downlink, a plurality ofbase stations transmits dedicated physical channels of the sameinformation. Therefore, interference increases. As a result, in adownlink, site diversity effects obtained through soft handover are lessthan those in an uplink. Accordingly, site selection diversitytransmission power control (SSDT) method has been proposed (“SSDT-SiteSelection Diversity Transmission Power Control for CDMA Forward Link”,H. Furukawa, K. Hamabe, and A. Ushirokawa: IEEE Journal on selectedareas in communications, vol. 18, no. 8, pp. 1546-1554, August 2000).SSDT method is a method where, among the base stations performing softhandover, only the base station with the largest received power or thelargest signal to interference power ratio of a received signal(hereafter referred to as the “most appropriate base station”) performsdata transmission and this most appropriate base station switches tohigh speed mode.

Considering the rapid spread of the Internet, increases in the dimensionof information, increases in the capacity of information, and thedevelopment of the next generation Internet in recent years, there is apressing need in mobile communication systems to develop a radio accessscheme capable of implementing an information transmission rateexceeding 2 Mbps. This need is considered to be increasing, especiallyin downlink traffic requiring high speed and a large capacity, includingdownloads of images, files, or moving images such as video from adatabase or Web site. Therefore, a high-speed packet transmissiontechnique suitable for high speed/large capacity traffic isindispensable.

From a background such as this, a proposal has been made to implementhigh speed packet transmission with a maximum information transmissionrate of 2.4 Mbps based on an IS-95 radio interface (“CDMA/HDR: ABandwidth-Efficient High-Speed Wireless Data Services for NomadicUsers”, P. Bender, P. Black, M. Grob, R. Padovani, N. Shindhushyana, andA. Viterbi: IEEE Communication Magazine, Vol. 38, no. 7, pp. 70-77, July2000). Additionally, in the 3rd generation partnership project (3GPP),implementation of high-speed packet transmission having a maximuminformation transmission rate of approximately 10 Mbps obtained byexpanding the W-CDMA radio interface has been studied.

Application of adaptive modulation and channel coding based on adaptiveradio link control such as channel coding, has been studied for suchhigh speed packet transmission, for example, as proposed in “Symbol Rateand Modulation Level-Controlled Adaptive Modulation/TDMA/TDD system forHigh-Bit-Rate Wireless Data Transmission”, T. Ue, S. Sampei, and N.Morinaga: IEEE Transactions VT, pp. 1134-1147, Vol. 47, no. 4, Nov.1988).

In adaptive modulation and channel coding based on adaptive radio linkcontrol, the data modulation level, spreading factor (SF), the number ofmulti-codes, and the channel coding rate are switched over according tothe propagation environment of a user in order to conduct high speeddata transmission -efficiently. For example, as for data modulation, thequadrature phase shift keying (QPSK) modulation used in the currentW-CDMA is switched over to a multi-level modulation having a higherefficiency, such as 8 PSK modulation, 16 quadrature amplitude modulation(QAM), or 64 QAM, as the propagation environment becomes favorable. As aresult, the maximum throughput of the communication system can beincreased.

As for the high speed packet transmission, application of the automaticrepeat request (ARQ) technique proposed in “Automatic-Repeat-RequestError Control Schemes”, (S. Lin, D. Costello, Jr., and M. Miller: IEEECommunications Magazine. Vol. 12, no. 12, pp. 5-17, Dec. 1984) has alsobeen studied.

The high-speed packet channel used for such high-speed packettransmission is a shared channel. The shared channel is used by aplurality of mobile stations. Therefore, the transmission power of theshared channel becomes significantly larger than that of the dedicatedphysical channel. The dedicated physical channel is a dedicated channelfor each mobile station.

Accordingly, in SSDT method, where it is always only the mostappropriate base station that is transmitting data, interference withother cells or sectors can be reduced in comparison with a soft handoverwhere there is a plurality of base stations performing data transmissionsimultaneously. As a result, with high-speed packet transmission,application of the SSDT method is studied in order to improve datathroughput in the end of cells and sectors, and enlarge the rangecapable of achieving target data throughput value.

However, in high-speed packet transmission that uses the high-speedpacket channel, each base station assigns the high-speed packet channelto a plurality of mobile stations. The high speed packet channel is ashared channel. Therefore, an opportunity of communication using ahigh-speed packet channel is assigned only to one, or a few mobilestations at any point in time.

Accordingly, even if the most appropriate base station is selected basedon the received power or signal to interference power ratio of thereceived signal, and the cell or sector covered by the most appropriatebase station is selected as well, the opportunity of communication isnot always assigned to the mobile station in that cell or sector uponreceiving assignment of the high speed packet channel. In particular,when there is a large traffic load in the selected cells or sectors, theopportunity of communication assigned to the mobile station in that cellor sector becomes significantly limited. In other words, the frequencywith which the base station grants the opportunity of communication tomobile stations drops. In addition, throughput of the mobile station towhich the opportunity of communication is not assigned drops.Furthermore, since the base station selects a cell or sector for aplurality of mobile stations based on the received power and signal tointerference power ratio in the mobile station, control load in the basestation increases. Therefore, the efficiency with which the base stationperforms control in order to communicate with the mobile stations alsodrops. As a result, in particular, data throughput performed by themobile stations located at the end of the cells or sectors drops.Accordingly, the range capable of achieving the target data throughputvalue cannot be enlarged.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to improve throughput ofcommunication performed by the mobile stations located at the end of thecommunication area, and to enlarge the range capable of achieving thetarget data throughput value.

A mobile station according to an aspect of the present inventioncomprises a reception unit configured to receive a signal transmittedfrom a base station; a selection unit configured to select acommunication area to perform communication from a plurality ofcommunication areas based on traffic load in the plurality ofcommunication areas where the reception unit can receive the signal; anda notification unit configured to notify the base station of thecommunication area selected by the selection unit.

The communication area means a radio area covered by the base station.The communication area is an area where the mobile station performscommunication with the base station by transmitting/receiving a signalwith the base station. The units of the communication area are: a “cell”that is a radio area covered by a single base station, and a “sector”that is obtained through further dividing the cell. The traffic load inthe communication area means the amount of communication performed inthe communication area. The traffic load may be represented by usingindexes, such as, the number of mobile stations that performcommunication with the base station in the communication area, the datavolume transmitted/received between the base station and mobile stationthat is represented by a utilization rate of a channel for performingdata transmission/reception, set between the base station and mobilestation, the data volume that is held in the base station and which isto be transmitted to the mobile station, and the number of mobilestations which are transmission destination of the data held in the basestation.

With such as the mobile station, the mobile station can detect thecommunication area capable of receiving a signal when the reception unitreceives a signal from the base station. The selection unit then selectsthe communication area for performing communication from thecommunication areas based on the traffic load in a plurality ofcommunication areas. Therefore, the mobile station may select thecommunication area to which the opportunity of communication is easilyassigned in consideration of the traffic load in the communication areaas the communication area for performing communication. Furthermore, thenotification unit notifies the base station of the selectedcommunication area. Therefore, the base station may controlcommunication with the mobile station by using the notifiedcommunication area. In other words, there is no need for the basestation to select the communication area for a plurality of mobilestations, and the control load of the base station can be distributed tothe mobile station. Accordingly, it is possible to improve controlefficiency for communication of the base station with the mobilestation.

As a result, it is possible to increase the opportunity of communicationassigned to the mobile station by the base station. Also, controlefficiency of the base station may be improved. As a result, it ispossible for the mobile station to increase data throughput performed bythe mobile station located at the end of the communication area. It ispossible for the mobile station to enlarge the range capable ofachieving the target data throughput value.

A mobile station according to another aspect of the present inventioncomprises a reception unit configured to receive a signal transmittedfrom a base station; a calculation unit configured to calculate a ratioof an average reception quality value over a short period to an averagereception quality value over a period longer than the short period as areception quality of a received signal received by the reception unit;and a selection unit configured to select a communication area toperform communication from a plurality of communication areas where thereception unit can receive the signal, based on the ratio calculated bythe calculation unit.

With such as the mobile station, the calculation unit calculates a ratioof the average reception quality value over a short period to theaverage reception quality value over a period longer than the shortperiod as the reception quality of received signal. The selection unitthen selects the communication area by using the ratio. The base stationselects the mobile station to perform communication, and assigns achannel to the selected mobile station. The base station selects themobile station to which a channel is assigned based on the ratio of theaverage reception quality value over a short period to the averagereception quality value over a period longer than the short period.Accordingly, the selection unit selects the communication area by usingthe same criterion as the criterion of the channel assignment.Accordingly, the mobile station may select the communication area towhich a channel is easily to be assigned, and the opportunity ofcommunication is easily assigned.

As a result, it is possible for the mobile station to increase theopportunity of communication assigned to the mobile station by the basestation. As a result, it is possible for the mobile station to increasedata throughput performed by the mobile station located on the end ofthe communication area. It is possible for the mobile station to enlargethe range capable of achieving the target data throughput value.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a diagram illustrating a communication system according to anembodiment of the present invention;

FIG. 2 is a block diagram illustrating a configuration of a base stationaccording to the embodiment of the present invention;

FIG. 3 is a block diagram illustrating a configuration of a mobilestation according to the embodiment of the present invention;

FIGS. 4A and 4B are graphs illustrating the received power of a receivedsignal according to the embodiment of the present invention;

FIG. 5 is a flowchart illustrating a procedure for a communicationmethod in the case of selecting a communication area based on thetraffic load included in a control signal according to the embodiment ofthe present invention;

FIG. 6 is a flowchart illustrating a procedure for a communicationmethod in the case of selecting a communication area by estimating thetraffic load according to the embodiment of the present invention;

FIG. 7 is a flowchart illustrating a procedure for a communicationmethod in the case of selecting a communication area based on thetraffic load and reception quality according to the embodiment of thepresent invention;

FIG. 8 is a flowchart illustrating a procedure for communication methodin the case of performing the directional beam transmission of a highspeed packet channel according to the embodiment of the presentinvention;

FIG. 9 is a flowchart illustrating a procedure for a communicationmethod in the case of performing scheduling using a channel assignmentalgorithm according to the embodiment of the present invention;

FIG. 10 is a flowchart illustrating a procedure for a communicationmethod in the case of performing the directional beam transmission of ahigh speed packet channel, and performing scheduling using a channelassignment algorithm according to the embodiment of the presentinvention;

FIG. 11 is a flowchart illustrating a procedure for a communicationmethod in the case of performing scheduling using a channel assignmentalgorithm, and selecting a communication area based on the traffic loadin the communication area according to the embodiment of the presentinvention;

FIG. 12 is a block diagram illustrating a configuration of a basestation according to a modified example of the present invention; and

FIG. 13 is a block diagram illustrating a procedure for a communicationmethod according to the modified example of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[Communication System]

An embodiment of the present invention will be described with referenceto the drawings. As shown in FIG. 1, a communication system 1 includes aplurality of base stations 2 a, 2 b, and 2 c, and a plurality of mobilestations 3. The base stations 2 a through 2 c and mobile stations 3transmit/receive data and communicate with each other.

(Base Station)

As shown in FIG. 2, the base station 2 a includes an antenna 21, a radiounit 22, a baseband processing unit 23, a buffer 24, a network interfaceunit 25, and a control unit 26. The base stations 2 b and 2 c aresubstantially the same as the base station 2 a. The base stations 2 athrough 2 c perform communication with a mobile station 3.

The antenna 21 transmits a packet and control signal input from theradio unit 22 to a mobile station 3. The antenna 21 inputs the packetand control signal received from the mobile station 3 to the radio unit22. The radio unit 22 transmits the packet and control signal input fromthe baseband processing unit 23 to the mobile station 3 via the antenna21. The radio unit 22 receives the packet and control signal transmittedfrom the mobile station 3 via the antenna 21 and inputs these to thebaseband processing unit 23.

The radio unit 22 also functions as a transmission unit that transmits acontrol signal including the traffic load in the communication areacovered by the base station 2 a to the mobile station 3 through acontrol channel. The radio unit 22 transmits control information to themobile station 3 through a downlink control channel. The downlinkcontrol channel is used to transmit a control signal, which includescontrol information from the base stations 2 a through 2 c to the mobilestation 3. The downlink control channel includes a downlink commoncontrol channel, a downlink shared control channel, and a common pilotchannel. The radio unit 22 also transmits data to the mobile stations 3through the high speed packet channel.

The downlink common control channel is a channel that is used totransmit a control signal including common control information to aplurality of mobile stations 3. Therefore, the downlink common controlchannel is common to a plurality of mobile stations 3, and is effectivefor transmitting the traffic load in a communication area, that isinformation that does not change for each mobile station 3. Accordingly,the radio unit 22 preferably transmits the traffic load in thecommunication area covered by the base stations 2 a through the downlinkcommon control channel.

The downlink shared control channel is a channel that transmits acontrol signal shared by a plurality of mobile stations 3. Therefore,each mobile station 3 is able to obtain control information transmittedthrough the downlink shared control channel. As a result, the downlinkshared control channel is effective for transmitting the traffic load inthe communication area required for all of the mobile stations 3.Accordingly, the radio unit 22 preferably transmits the traffic load inthe communication area covered by the base stations 2 a through thedownlink shared control channel. Note that the radio unit 22 transmitsthe traffic load in the communication area to the mobile station 3 inaccordance with the control by the control unit 26 through the controlchannel, such as the downlink common control channel or downlink sharedcontrol channel.

The common pilot channel is a specific channel for transmitting a commonpilot signal to each mobile station 3. The pilot signal is used when themobile station 3 performs channel estimation, and is a known signal inthe mobile station 3. The high speed packet channel is a channel, whichis used to transmit a packet including data from the base stations 2 athrough 2 c to the mobile station 3. The high speed packet channel is adownlink shared channel, which is shared by a plurality of mobilestations 3. Note that the radio unit 22 transmits a packet to the mobilestation 3 through the high speed packet channel in accordance with thechannel assignment by the control unit 26.

Note that directional beam transmission may be performed to transmit apacket to a predetermined direction when the radio unit 22 transmits apacket through a high speed packet channel. In this case, two kinds ofcommon pilot channels—a primary common pilot channel and a secondarycommon pilot channel—are used as the common pilot channel. The primarycommon pilot channel is performed non-directional beam transmission of apilot signal. The secondary common pilot channel is performeddirectional beam transmission of a pilot signal.

The radio unit 22 also functions as a reception unit that receives acontrol signal transmitted from the mobile station 3 via the uplinkchannel. The control signal transmitted from the mobile station 3includes data that indicates the communication area selected by themobile station 3. The data indicating the communication areas includesidentification data specific to the base stations 2 a through 2 c, whichcan identify the base stations 2 a through 2 c that cover thecommunication area, and identification data specific to thecommunication area, which can identify the communication area such asthe cell or sector. The identification data for the base stations 2 athrough 2 c includes addresses of the base stations 2 a through 2 c, andbase station IDs. The identification data for the communication areaincludes cell IDs and sector IDs.

The radio unit 22 receives a packet including data and a control signal,which includes control information from the mobile stations 3 via theuplink channel. The uplink channel is the channel that is used totransmit a control signal and a packet from the mobile stations 3 to thebase stations 2 a through 2 c. The uplink channel includes a dedicatedchannel that is individually used by each mobile station 3 and a sharedchannel shared by a plurality of mobile stations 3. Furthermore, thededicated channel includes a dedicated physical data channel, and adedicated physical control channel. The shared channel and dedicatedphysical data channel are used for transmitting the control signal andpacket. The dedicated physical control channel is used for transmittingthe control signal.

The baseband processing unit 23 performs signal processing for a packetand a control signal to be transmitted to the mobile station 3, andinputs them to the radio unit 22. For example, the baseband processingunit 23 performs signal processing, such as channel coding, datamodulation, and spreading modulation, on the packet and control signal.The baseband processing unit 23 obtains the packet from the buffer 24.The control signal is input from the control unit 26 to the basebandprocessing unit 23. The baseband processing unit 23 also performs signalprocessing for the packet and control signal received by the radio unit22. For example, the baseband processing unit 23 performs signalprocessing, such as channel decoding, data demodulation, anddespreading, on the packet and control signal. The baseband processingunit 23 stores the received packet in the buffer 24. In addition, thebaseband processing unit 23 inputs the received control signal to thecontrol unit 26.

The buffer 24 holds a packet to be transmitted to the mobile station 3and the packet transmitted from the mobile station 3. The networkinterface unit 25 obtains from the buffer 24 the packet transmitted fromthe mobile station 3. The network interface unit 25 transmits thatpacket via the network 4. The network interface unit 25 obtains thepacket to be transmitted to the mobile stations 3 via the network 4. Thenetwork interface unit 25 stores that packet in the buffer 24.

The control unit 26 controls the radio unit 22, baseband processing unit23, and network interface unit 25. The control unit 26 instructs theradio unit 22 to transmit the traffic load in the communication areathrough a control channel, such as a downlink common control channel ordownlink shared control channel. More specifically, the control unit 26generates a control signal, which includes the traffic load. The controlunit 26 then inputs the generated control signal to the basebandprocessing unit 23. The radio unit 22 obtains the control signalgenerated by the control unit 26 from the baseband processing unit 23.And the radio unit 22 transmits that control signal through the controlchannel, such as the downlink common control channel or downlink sharedcontrol channel.

The control unit 26 generates a control signal including, for example,the number of mobile stations 3 performing communication with the basestation 2 a in the communication area, the utilization rate of the highspeed packet channel, which is the data volume transmitted/receivedbetween the base station 2 a and the mobile station 3, the total numberof packets to be transmitted to the mobile station 3, which are held inthe buffer 24 by the base station 2 a, and the number of mobile stations3 which are transmission destinations of packets held in the buffer 24by the base station 2 a, as the traffic load.

The number of mobile stations 3 performing communication with the basestation 2 a in the communication area is the number of mobile stations 3connected to the radio unit 22. The utilization rate of the high speedpacket channel is the utilization rate of the high speed packet channelto be transmitted by the radio unit 22. Therefore, these are known inthe radio unit 22. Accordingly, the radio unit 22 inputs to the controlunit 26 the number of mobile stations 3 connected to the radio unit 22and the utilization rate of the high speed packet channel to betransmitted by the radio unit 22. The control unit 26 then generates thecontrol signal including the number of mobile stations 3 and theutilization rate of the high speed packet channel input from the radiounit 22, as the traffic load.

The control unit 26 references the buffer 24, and obtains the totalnumber of packets to be transmitted to the mobile stations 3 held in thebuffer 24 by the base stations 2 a, and the number of mobile stations 3which are the transmission destinations of the packet. The control unit26 generates the control signal including the obtained total number ofpackets and the number of mobile stations 3 which are the transmissiondestinations of the packets, and the traffic load.

The control unit 26 also controls to communicate with the mobile station3 in the communication area selected by the mobile station 3. A controlsignal is input to the control unit 26 from the baseband processing unit23. As a result, the control unit 26 obtains the control signal receivedby the radio unit 22 from the mobile station 3 via the basebandprocessing unit 23. The control unit 26 obtains data from the controlsignal, which indicates the communication area selected by the mobilestation 3 included in that control signal. The control unit 26 controlsthe baseband processing unit 23 and radio unit 22 to transmit the packetto the mobile station 3 via the high speed packet channel, when the dataindicating the communication area indicates the communication area thatis covered by that base station 2 a.

More specifically, the control unit 26 assigns the high speed packetchannel for packet transmission to that mobile station 3. The controlunit 26 inputs the result of the high speed packet channel assignment tothe radio unit 22, and instructs the radio unit 22 to transmit thepacket in accordance with the channel assignment. The control unit 26instructs the baseband processing unit 23 to obtain the packet to themobile station 3 from the buffer 24, perform signal processing, andinput the packet to the radio unit 22. The baseband processing unit 23then performs signal processing of the packet to the mobile stations 3and inputs the packet to the radio unit 22. The radio unit 22 transmitsthe packet to the mobile stations 3 through the high speed packetchannel in accordance with the channel assignment instructed by thecontrol unit 26. Note that the control unit 26 also assigns the uplinkchannel for packet transmission from the mobile station 3 to the basestations 2 a.

Meanwhile, the control unit 26 determines not to transmit the packet tothe mobile station 3 when the data indicating the communication areadoes not indicate the communication area covered by the base station 2a. In this case, the control unit 26 performs no specific operationtoward the baseband processing unit 23 and radio unit 22.

(Mobile Station)

As shown in FIG. 3, the mobile station 3 includes an antenna 31, a radiounit 32, a baseband processing unit 33, an input unit 34, an output unit35, a card interface unit 36, a buffer 37, and a control unit 38. Themobile station 3 communicates with the base stations 2 a through 2 c inthe communication area, such as cells or sectors, covered by the basestations 2 a through 2 c.

The antenna 31 transmits a packet and control signal input from theradio unit 32 to the base stations 2 a through 2 c. The antenna 31receives the packet and control signal from the base stations 2 athrough 2 c and inputs them to the radio unit 32. The radio unit 32transmits the packet and control signal input from the basebandprocessing unit 33 to the base stations 2 a through 2 c via the antenna31.

The radio unit 32 functions as a notification unit that notifies thebase stations 2 a through 2 c, of a communication area to performcommunication, which is selected by the mobile station 3. Morespecifically, the radio unit 32 notifies of data that indicates theselected communication area, through an uplink channel such as adedicated channel or shared channel. The radio unit 32 transmits, forexample, a control signal including the address of the base stations 2 athrough 2 c, base station ID, cell ID, or sector ID to the base stations2 a through 2 c. The radio unit 32 multiplexes the control signalincluding data that indicates the communication area, with the packetincluding other control signals and data in the uplink channel, and thentransmits them. Time multiplexing or code multiplexing may be used tomultiplex the control signal. In accordance with the control unit 38,the radio unit 32 notifies the base stations 2 a through 2 c that thedata indicates the selected communication area. Note that the radio unit32 transmits a packet, which includes data to the base stations 2 athrough 2 c through the shared channel or the dedicated physical datachannel.

The radio unit 32 functions as a reception unit that receives a controlsignal and a signal including a packet transmitted from the basestations 2 a through 2 c via the antenna 31. The radio unit 32 inputsthe received signal to the baseband processing unit 33. The radio unit32 receives the control signal including the traffic load via thedownlink common control channel or downlink shared control channel. Theradio unit 32 also receives a pilot signal through the common pilotchannel. The radio unit 32 receives a packet including data through thehigh speed packet channel. The radio unit 32 measures the received powerwhen the control channel and high speed packet channel transmitted fromthe base stations 2 a through 2 c are received, and inputs the receivedpower to the control unit 38.

The baseband processing unit 33 obtains data to be transmitted to thebase stations 2 a through 2 c from the input unit 34 and card interfaceunit 36. The baseband processing unit 33 generates a packet includingthe obtained data. The baseband processing unit 33 performs signalprocessing on the generated packet and control signal, and inputs themto the radio unit 32. For example, the baseband processing unit 33performs signal processing, such as channel coding, data modulation, andspreading modulation, for the packet and control signal. The basebandprocessing unit 33 stores the generated packet in the buffer 37, andobtains the packet from the buffer 37 for transmission. The controlsignal is input from the control unit 38 to the baseband processing unit33.

The baseband processing unit 33 also performs signal processing on thepacket and control signal received by the radio unit 32 from the basestations 2 a through 2 c. For example, the baseband processing unit 33performs signal processing, such as channel decoding, data demodulation,and despreading, for the received data packet and control signal. Thebaseband processing unit 33 outputs the data included in the receivedpacket to the output unit 35. The baseband processing unit 33 inputs thedata included in the received packet to the card interface unit 36. Inaddition, the baseband processing unit 33 inputs the received controlsignal to the control unit 38.

The input unit 34 inputs data to be transmitted from the mobile station3 to the baseband processing unit 33. The data received by the mobilestation 3 is input to the output unit 35 from the baseband processingunit 33, and the output unit 35 outputs the data. The card interfaceunit 36 obtains data to be transmitted from the mobile station 3 from anexternal device, such as a personal computer, via a specific datatransmission card, such as a PC card. The card interface unit 36 inputsthe obtained data to the baseband processing unit 33. The data receivedby the mobile station 3 is input to the card interface unit 36 from thebaseband processing unit 33. The card interface unit 36 inputs datareceived by the mobile stations 3 to the external device via the card.The buffer 37 holds the packet to be transmitted to the base stations 2a through 2 c.

The control unit 38 controls the radio unit 32 and baseband processingunit 33. The control unit 38 functions as a selection unit that selectsthe communication area where the mobile station 3 performscommunication. The control unit 38 selects the communication area toperform communication based on the traffic load in a plurality ofcommunication areas. For example, the control unit 38 selects acommunication area with the least traffic load as the communication areato perform communication.

The control unit 38 may detect, for example, the traffic load in eachcommunication area through the following method. A control signal isinput to the control unit 38 from the baseband processing unit 33. As aresult, the control unit 38 obtains a control signal, which is receivedby the radio unit 32 via the baseband processing unit 33. The controlunit 38 may detect the traffic load by obtaining the traffic load ineach communication area included in the control signal. The control unit38 obtains as the traffic load in the communication area, for example,the number of mobile stations 3 performing communication with the basestations 2 a through 2 c in each communication area, the utilizationrate of the high speed packet channel, the total number of packets to betransmitted to the mobile stations 3 are held by the base stations 2 athrough 2 c, and the number of mobile stations 3 which are transmissiondestinations of packets held by the base stations 2 a through 2 c.Accordingly the control unit 38 may select the communication area basedon the traffic load included in the control signal.

The control unit 38 also functions as an estimation unit that estimatesthe traffic load in the plurality of communication areas based on thereceived power of the signal received by the radio unit 32. The signalis transmitted from the base stations 2 a through 2 c. The high speedpacket channel that transmits a packet including data, and a pluralityof control channels that transmit a control signal including controlinformation are transmitted from the base stations 2 a through 2 c. FIG.4A illustrates received power when the radio unit 32 receives thechannel transmitted from the base station 2 a. FIG. 4B illustratesreceived power when the radio unit 32 receives the channel transmittedfrom the base station 2 b. In FIGS. 4A and 4B, the vertical axisindicates the received power received by the mobile stations 3, and thehorizontal axis indicates time.

The radio units 22 of the base stations 2 a and 2 b transmit the highspeed packet channel at an extremely high transmission power. The radiounits 22 transmit the control channel at a low transmission power.Therefore, when there are not packets to be transmitted in the basestations 2 a and 2 b, and the traffic load in the communication areascovered by these base stations 2 a and 2 b is little, the radio unit 22does not transmit the high speed packet channel. Therefore, the radiounit 32 of the mobile stations 3 receives only the control channeltransmitted at the low transmission power. Accordingly, the receivedpower of the channel received by the mobile stations 3 is only thereceived power 10, when the control channel is received, and it becomeslow.

Meanwhile, when there are packets to be transmitted in the base stations2 a and 2 b, and the traffic load in the communication areas covered bythese base stations 2 a and 2 b is large, the radio unit 22 transmitsthe high speed packet channel. Therefore, the radio unit 32 of themobile station 3 receives the control channel transmitted at the lowtransmission power and the high speed packet channel transmitted at thehigh transmission power. Accordingly, the received power of the channelreceived by the mobile station 3 is found by adding the received power10 when the control channel is received and the received power 9 whenthe high speed channel is received, and it becomes extremely high.

Therefore, the control unit 38 may obtain the received power when thechannel is received from the radio unit 32, and estimate the trafficload in the communication area by using that received power. Morespecifically, since it seems that no high speed packet channel istransmitted when the received power is low, the control unit 38 mayestimate that the traffic load in the communication area is little.Meanwhile, since it seems that high speed packet channel is transmittedwhen the received power is high, the control unit 38 may estimate thatthe traffic load in the communication area is large.

Alternatively, the control unit 38 calculates the ratio of the receivedpower when the common pilot channel is received included in the receivedpower 10 when the control channel is received, and the received powerwhen other channel (channel other than the common pilot channel) isreceived. The ratio of the high received power 9 when the high speedpacket channel is received and the low received power when the pilotchannel is received may be large. Therefore, the control unit 38determines that no high speed packet channel is included in the channelother than the pilot channel when the ratio of the received power whenthe common pilot channel is received, and the received power when theother channel is received is small. The control unit 38 then estimatesthat the traffic load in the communication area is little.

Meanwhile, the control unit 38 determines that the high speed packetchannel is included in the channel other than the pilot channel when theratio of the received power when the common pilot channel is receivedand the received power when the other channel is received is large. Thecontrol unit 38 then estimates that the traffic load in thecommunication area is large.

In the case of FIG. 4A, at times 5 and 7, since the received power ofthe mobile station 3 becomes the total of the received power 10 when thecontrol channel is received plus the received power 9 when the highspeed channel is received, so it is very large. In addition, the ratioof the received power when the common pilot channel is received that isincluded in the received power 10 when the control channel is received,and the received power when the channel other than the common pilotchannel is received including the received power 9 when the high speedpacket channel is received is also large. Therefore, the control unit 38is able to estimate that the traffic load in the communication areacovered by the base station 2 a is large.

Meanwhile, in the case of FIG. 4B, at times 5 and 7, the received powerwhen the mobile stations 3 receive the channel from the base station 2 bis only the received power 10 when the control channel is received, soit is small. In addition, the ratio of the received power when thecommon pilot channel is received included in the received power 10 whenthe control channel is received and the received power when the channelother than the common pilot channel included in the received power 10 isalso small. Therefore, the control unit 38 is able to estimate that thetraffic load in the communication area covered by the base station 2 bis little. Accordingly, at times 5 and 7, the control unit 38 is able toselect a communication area of the base station 2 b where the trafficload is estimated as little, as the communication area for performingcommunication.

Conversely, at times 6 and 8 (see FIG. 4A), the control unit 38 is ableto estimate that the traffic load in the communication area covered bythe base station 2 a is little. The control unit 38 is then able toselect a communication area in the base station 2 a where the trafficload is estimated as little, as the communication area for performingcommunication.

In this manner, the control unit 38 of the mobile station 3 is able toestimate the traffic load in a plurality of communication areas based onthe received power of a signal received by the radio unit 32. Thecontrol unit 38 is then able to select the communication area based onthe estimated traffic load. Therefore, the mobile station 3 is able toestimate the traffic load and select the communication area withoutusing the control signal. Accordingly, the control load of the basestations 2 a through 2 c may be further reduced. In addition, radioresources are not consumed for transmitting the control signal.Therefore, the radio resources may be utilized for data transmission. Asa result, the throughput of the communication performed by the mobilestation 3 further increases.

The control unit 38 may select the communication area for performingcommunication based on the reception quality of the received signalreceived by the radio unit 32 in addition to the traffic load in thecommunication area. For example, the control unit 38 finds the receptionquality of the common pilot channel received by the radio unit 32. Morespecifically, the control unit 38 obtains the received power when thecommon pilot channel is received, from the radio unit 32. The controlunit 38 then finds the reception quality, such as the received power ofthe pilot signal, and the signal to interference power ratio of thepilot signal based on the received power. Note that the receptionquality found may be the received power or signal to interference powerratio at any point in time, or it may be an average of the receivedpower and signal to interference power ratio.

The control unit 38 holds the predetermined threshold value of thereception quality from the most preferable reception quality. Thecontrol unit 38 selects from the communication areas that have the mostpreferable reception quality to the communication areas that have areception quality within the threshold value as communication areacandidates for performing communication. The control unit 38 selects thecommunication area with the least traffic load in the communication areafrom the selected communication area candidates.

Alternatively, the control unit 38 may hold the predetermined thresholdvalue of the traffic load from the least traffic load. In this case, thecontrol unit 38 selects from the communication area with the leasttraffic load to the communication area with the traffic load within thethreshold value as communication area candidates for performingcommunication. The control unit 38 selects the communication area thathas the most preferable reception quality in the communication area fromthe selected communication area candidates.

In this manner, the control unit 38 may select the communication area inconsideration of not only the traffic load in the communication area butalso the reception quality. For example, the control unit 38 may selectthe communication area with little traffic load from the communicationareas that have some degree of reception quality. The control unit 38may also select the communication area that has a preferable receptionquality from the communication areas that have little traffic load, tosome degree. Therefore, the mobile station 3 may select a communicationarea where the traffic load is little, an opportunity of communicationcan be easily assigned, and the reception quality is preferable.Accordingly, both the opportunity of communication assigned to themobile stations 3 from the base stations 2 a through 2 c and theprobability that the mobile station 3 has a successful datatransmission/reception may be improved. As a result, the throughput ofthe communication performed by the mobile station 3 may be furtherimproved.

Note that when the radio unit 22 of the base stations 2 a through 2 cperforms directional beam transmission for transmitting packets throughthe high speed packet channel, two kinds of common pilot channels, thatis, the primary common pilot channel and secondary common pilot channel,are used. The primary common pilot channel is performed non-directionalbeam transmission. Therefore, the reception quality found by using theprimary common pilot channel does not match with the reception qualityof the high speed packet channel that is performed directional beamtransmission. Accordingly, the reception quality found by using theprimary common pilot channel is not preferable as the reception qualityto be used in selecting the communication area for performingcommunication.

Meanwhile, the secondary common pilot channel is performed directionalbeam transmission in the same direction as the high speed packetchannel. Accordingly, the reception quality found by using the secondarycommon pilot channel matches with the reception quality of the highspeed packet channel that is performed directional beam transmission.Therefore, the control unit 38 preferably selects the communication areafor performing communication by using the reception quality found basedon the secondary common pilot channel. As a result, it is possible toselect the communication area based on the reception quality inconsideration of the fact that the high speed packet channel isperformed directional beam transmission.

The control unit 26 of the base stations 2 a through 2 c assigns achannel for packet transmission from the base stations 2 a through 2 cto the mobile station 3, and for packet transmission from the mobilestation 3 to the base stations 2 a through 2 c. The channel assignmentmethod includes a method of using a channel assignment algorithm thatselects the mobile station 3 with the largest ratio of the averagereception quality value of the mobile station 3 over a short period tothe average reception quality value of the mobile station 3 over aperiod longer than the short period. This channel assignment method iscalled “proportional fairness scheduling” (see “Data Throughput ofCDMA-HDR a High Efficiency-High Data Rate Personal CommunicationWireless System”, A. Jalali, R. Padovani, R. Pankaj: Proc. of IEEEVTC2000—Spring, pp. 1854-1858, May 2000).

When the control unit 26 of the base stations 2 a through 2 c usesproportional fairness scheduling for channel assignment, the controlunit 38 may select the communication area for performing communicationbased on the ratio of the average reception quality value of a signalreceived by the radio unit 32 over a short period to the averagereception quality value over a period longer than the short period. Inthis case, the control unit 38 also functions as a calculating unit thatcalculates the ratio of the average reception quality value over a shortperiod to the average reception quality value over a period longer thanthat short period as the reception quality of a signal received by theradio unit 32.

The control unit 38 calculates, for example, the ratio of the averagereception quality value over the short period to the average receptionquality value over the long period as the reception quality of thecommon pilot channel received by the radio unit 32. More specifically,the control unit 38 obtains the received power with which the commonpilot channel is received from the radio unit 32. The control unit 38then finds the reception quality, such as the received power of thereceived pilot signal and signal to interference power ratio of thereceived pilot signal, based on that received power. The control unit 38calculates the average reception quality value over the short period tothe average reception quality value over the long period, and calculatesthe ratio thereof based on the found reception quality. The control unit38 then selects the communication area with the highest ratio as thecommunication area for performing communication.

Note that in the case where the radio unit 22 of the base stations 2 athrough 2 c performs directional beam transmission to transmit a packetusing a high speed packet channel, it is preferable that the averagereception quality value be calculated using the secondary common pilotchannel that is performed to directional beam transmission in the samedirection as the high speed packet channel, and then the ratio of thetwo (the average reception quality value over a short period to theaverage reception quality value over a long period) be calculated.Accordingly, it is possible to select the communication area forperforming communication based on the reception quality in considerationof the fact that the high speed packet channel is performed todirectional beam transmission.

The control unit 38 may also select the communication area inconsideration of the traffic load in the communication area even whenselecting the communication area based on the ratio of the averagereception quality value over a short period to the average receptionquality value over a long period. For example, the control unit 38 holdsthe predetermined threshold value of the ratio found from the largestvalue of the ratio of the average reception quality value over a shortperiod to the average reception quality value over a long period of asignal. The control unit 38 selects candidate communication areas forperforming communication from the communication area with the largestratio found to the communication area having the value of the ratiowithin the threshold value. The control unit 38 selects thecommunication area with the least traffic load in the communication areafrom the selected communication area candidates.

Alternatively, the control unit 38 may hold a predetermined traffic loadthreshold value from the lowest traffic load. In this case, the controlunit 38 selects candidate communication areas for performingcommunication, from the communication area with the least traffic loadto the communication area with a traffic load within the thresholdvalue. The control unit 38 selects from the selected communication areacandidates, the communication area where the ratio of the averagereception quality over a short period to the average reception qualityvalue over a long period is the largest.

The control unit 38 generates a control signal including data indicatingthe selected communication area. The control unit 38 instructs the radiounit 32 to transmit the control signal including data indicating theselected communication area through an uplink channel, such as a commonchannel or dedicated channel. More specifically, the control unit 38generates a control signal including data indicating the selectedcommunication area, and inputs the control signal to the basebandprocessing unit 33. The radio unit 32 obtains the control signalgenerated by the control unit 38 via the baseband processing unit 33.The radio unit 32 multiplexes that control signal in the uplink channel(common channel or dedicated channel) and transmits them to the basestations 2 a through 2 c.

[Communication Method]

Next, a communication method using such a communication system 1 isdescribed. Although omitted from FIG. 5 through FIG. 11, the basestation 2 c performs substantially the same operation as the basestations 2 a and 2 b. In FIG. 5 through FIG. 11, the case where a cellis the unit of communication area is described as an example.

To begin with, a communication method where the mobile station 3 selectsthe communication area based on the traffic load in the communicationarea included in the control signal transmitted through the controlchannel from the base stations 2 a and 2 b is described using FIG. 5. Asshown in this figure, the radio units 22 of the base stations 2 a and 2b transmit the control signal including the traffic load of the cellsrespectively covered by the base stations 2 a and 2 b, to the mobilestation 3 (S101). The base stations 2 a and 2 b transmit the controlsignal using a downlink common control channel or downlink sharedcontrol channel.

The radio unit 32 of the mobile station 3 receives the control signalincluding the traffic load transmitted from the base stations 2 a and 2b, respectively (S102). The mobile station 3 receives the control signalthrough the downlink common control channel or downlink shared controlchannel. The control unit 38 of the mobile station 3 obtains the controlsignal, which is received by the radio unit 32 via the basebandprocessing unit 33. The control unit 38 obtains the traffic load in eachcell from the control signal. The control unit 38 then selects a cellwith the least traffic load as the cell for performing communication. Inthis case, since the unit of communication area is a cell, the mobilestation 3 selects the base station that covers the selected cell (S103).The control unit 38 of the mobile station 3 generates a control signalincluding the identification data of the selected base station. Theradio unit 32 of the mobile station 3 multiplexes the control signal inthe uplink channel, and transmits the control signal to the respectivebase stations 2 a through 2 c (S104).

In the base stations 2 a and 2 b, the radio units 22 receive the controlsignal including the identification data of the base station selected bythe mobile station 3, respectively. The radio unit 22 receives thecontrol signal transmitted from the mobile stations 3 through the uplinkchannel. The baseband processing unit 23 then demodulates data (S105).The control units 26 of respective base stations 2 a and 2 b determinewhether the identification data included in the control signal indicatesits own base station. In other words, whether a cell covered by basestation 2 a or 2 b has been indicated is determined (S106).

In step (S106), the control units 26 of the respective base stations 2 aand 2 b decide to transmit a packet when the identification data of thebase station included in the control signal indicates its own basestation. The control unit 26 assigns the high speed packet channel forpacket transmission to the mobile station 3. The radio unit 22 thentransmits the packet (S107).

Meanwhile, in step (S106), the control units 26 of the respective basestations 2 a and 2 b decide not to transmit a packet when theidentification data of the base station included in the control signaldoes not indicate its own base station (S108). In this manner, thepacket is transmitted only from the base station, which covers the cellselected by the mobile station 3.

Next, the case where the mobile station 3 estimates the traffic load inthe communication area and selects the communication area is described.As shown in FIG. 6, to begin with, the radio units 22 of the basestations 2 a and 2 b transmit a channel such as a downlink controlchannel or high speed packet channel to the mobile station 3 (S201).

The radio unit 32 of the mobile station 3 measures the received power ofthe channel received from each of the base stations 2 a and 2 b, andinputs the result to the control unit 38. Alternatively, the controlunit 38 calculates the ratio of the received power when the common pilotchannel is received and the received power when other channel (a channelother than the common pilot channel) is received, based on the receivedpower input by the radio unit 32 (S202).

For a cell with a low received power, or a cell where the ratio of thereceived power when the common pilot channel is received and thereceived power when the other channel is received is small, the controlunit 38 estimates that the traffic load of that cell is little. Thecontrol unit 38 then selects the cell where the traffic load isestimated as little, as the cell for performing communication. In thiscase, since the communication area unit is a cell, the mobile station 3selects the base station that covers the selected cell (S203). Step(S204) through step (S208) are substantially the same as step (S104)through step (S108) shown in FIG. 5.

Next, the case where the mobile station 3 selects the communication areabased on the traffic load in the communication area and the receptionquality of the signal is described. To begin with, as shown in FIG. 7,the radio units 22 of the base stations 2 a and 2 b transmit the controlsignal including the traffic load of a cell respectively covered by thebase stations 2 a and 2 b to the mobile station 3 (S301). The radio unit22 transmits the control signal through a downlink common controlchannel or downlink shared control channel. The radio units 22 of thebase stations 2 a and 2 b also transmit the pilot signal to the mobilestation 3 through the common pilot channel (S302).

The radio unit 32 of the mobile station 3 receives the common pilotchannel from the base stations 2 a and 2 b, respectively. The radio unit32 measures the received power of the received common pilot channel, andinputs the result to the control unit 38. The control unit 38 finds thereception quality, such as the received power, or signal to interferencepower ratio in the cell covered by each of the base stations 2 a and 2b, by using the pilot signal transmitted through the common pilotchannel (S303). The control unit 38 selects the base station candidatesto cover the cell performing communication, from the base station thatcovers the cell that has the most preferable reception quality, to thebase station that covers the cell that has a reception quality that iswithin the threshold value(S304). The control unit 38 selects the cellwith the least traffic load included in the control signal transmittedin step (S301) from the selected base station candidates. In this case,since the communication area unit is a cell, the mobile station 3selects the base station covering the selected cell (S305). Step (S306)through step (S310) are substantially the same as step (S104) throughstep (S108) shown in FIG. 5.

Next, the case where the mobile station 3 selects the communication areabased on the traffic load in the communication area and the receptionquality of the signal when the base station performs directional beamtransmission is described. To begin with, as shown in FIG. 8, the radiounits 22 of the base stations 2 a and 2 b transmit the control signalincluding the traffic load of a cell to the mobile station 3 (S401). Theradio units 22 of the base stations 2 a and 2 b also transmit the pilotsignal to the mobile station 3 through the primary common pilot channeland secondary common pilot channel (S402).

The radio unit 32 of the mobile station 3 receives the primary commonpilot channel and secondary common pilot channel from the respectivebase stations 2 a and 2 b. The radio unit 32 measures the received powerof the secondary common pilot channel performed directional beamtransmission, and inputs the result to the control unit 38. The controlunit 38 finds the reception quality, such as the received power or thesignal to interference power ratio, by using the pilot signaltransmitted through the secondary common pilot channel from therespective base stations 2 a and 2 b (403). The control unit 38 selectsthe base station candidates to cover the cell performing communication,from the base station that covers the cell that has the most preferablereception quality to the base station that covers the cell that hasreception quality found by using the secondary common pilot channelwithin the threshold value (S404). Step (S405) through step (S410) aresubstantially the same as step (S305) through step (S310) shown in FIG.7.

Next, the case where the base stations 2 a and 2 b perform proportionalfairness scheduling using a channel assignment algorithm is described.To begin with, as shown in FIG. 9, the radio units 22 of the basestations 2 a and 2 b transmit the pilot signal to the mobile stations 3through the common pilot channel (S501). The radio unit 32 of the mobilestation 3 receives the common pilot channel from the respective basestations 2 a and 2 b. The radio unit 32 measures the received power ofthe received common pilot channel, and inputs the result to the controlunit 38. The control unit 38 finds the reception quality, such as thereceived power or signal to interference power ratio, by using the pilotsignal transmitted through the common pilot channel from the respectivebase stations 2 a and 2 b (S502).

The control unit 38 finds the average reception quality value over ashort period (S503), and finds the average reception quality value overa period longer than the short period (S504), based on the foundreception quality. The control unit 38 calculates the ratio of theaverage reception quality value over a short period to the averagereception quality value over a long period, found in steps (S503) and(S504). The control unit 38 selects the cell with the largest ratio. Inthis case, since the communication area unit is a cell, the mobilestation 3 selects the base station that covers the selected cell (S505).Step (S506) through step (S510) are substantially the same as step(S104) through step (S108) shown in FIG. 5.

Next, the case where the base stations 2 a and 2 b perform directionalbeam transmission of the high speed packet channel and performproportional fairness scheduling is described. To begin with, as shownin FIG. 10, the radio units of the base stations 2 a and 2 b transmitthe pilot signal to the mobile station 3 through the primary commonpilot channel and secondary common pilot channel (S601). The radio unit32 of the mobile station 3 receives the primary common pilot channel andsecondary common pilot channel from the respective base stations 2 a and2 b. The radio unit 32 measures the received power of the secondarycommon pilot channel performed directional beam transmission as with thehigh speed packet channel, and inputs the result to the control unit 38.The control unit 38 finds the reception quality, such as the receivedpower or the signal to interference power ratio, by using the pilotsignal transmitted through the secondary common pilot channel from therespective base stations 2 a and 2 b (S602).

The control unit 38 finds the average reception quality value over ashort period (S603), and finds the average reception quality value overa period longer than the short period (S604), based on the receptionquality found by using the secondary common pilot channel. The controlunit 38 calculates the ratio of the average reception quality value overa short period to the average reception quality value over a long periodof the secondary common pilot channel found in steps (S603) and (S604).The control unit 38 then selects the cell with the largest ratio. Inthis case, since the communication area unit is a cell, the mobilestation 3 selects the base station that covers the selected cell (S605).Step (S606) through step (S610) are substantially the same as step(S104) through step (S108) shown in FIG. 5.

Next, the case where the mobile station 3 selects the communication areain consideration of the traffic load in the communication area when thebase stations 2 a and 2 b perform proportional fairness scheduling isdescribed. To begin with, as shown in FIG. 11, the radio units 22 of thebase stations 2 a and 2 b transmit the control signal including thetraffic load of the cell covered by each of the base stations 2 athrough 2 c to the mobile station 3 through the downlink common controlchannel or downlink shared control channel (S701). The radio units 22 ofthe base stations 2 a and 2 b also transmit the pilot signal to themobile station 3 through the common pilot channel (S702).

The radio unit 32 of the mobile station 3 receives the control signalincluding the traffic load. The control unit 38 of the mobile station 3obtains the traffic load in each cell from the control signal. Thecontrol unit 38 then selects base station candidates to cover the cellperforming communication from the base station that covers the cell withthe least traffic load, to the base station that covers cell with atraffic load within the threshold value (S703). The radio unit 32measures the received power of the common pilot channel received fromthe base station for the base station candidates selected by the controlunit 38, and inputs the result to the control unit 38. The control unit38 finds the reception quality, such as the received power or signal tointerference power ratio in the cell covered by each base stationcandidate, by using the pilot signal transmitted through the commonpilot channel (S704).

The control unit 38 finds the average reception quality value over ashort period (S705), and finds the average reception quality value overa long period (S706), based on the reception quality found for the cellcovered by the base station candidates. The control unit 38 calculatesthe ratio of the average reception quality value over a short period tothe average reception quality value over a long period for the cellcovered by the base station candidates found in steps (S705) and (S706).The control unit 38 selects the cell with the largest ratio. In thiscase, since the communication area unit of is a cell, the mobile station3 selects the base station that covers the selected cell (S707). Step(S708) through step (S712) are substantially the same as step (S104)through step (S108) shown in FIG. 5.

With such as the communication system 1, the base stations 2 a through 2c, the mobile station 3, and the communication method, the mobilestation 3 is able to detect the communication areas capable of receivinga signal, by the radio unit 32 in the mobile station 3 receives a signaltransmitted from the radio units 22 of base stations 2 a through 2 c.The control unit 38 then selects the communication area for performingcommunication from those communication areas based on the traffic loadin that plurality of communication areas. Therefore, the mobile station3 can select a communication area in which an opportunity ofcommunication is easily assigned in consideration of the traffic load inthat communication area. Furthermore, traffic load can be distributedamong the respective communication areas. The radio unit 32 notifies thebase stations 2 a through 2 c of the control signal including thecommunication area for communication selected by the control unit 38, bymultiplexing the signal in the uplink channel and transmitting thecontrol signal to the base stations 2 a through 2 c. Therefore, thecontrol units 26 of the base stations 2 a through 2 c may control tocommunicate with the mobile station 3 in the communication areanotified. In this manner, it is not necessary for the base stations 2 athrough 2 c to select for the plurality of mobile stations 3, thecommunication area where each mobile station 3 is to performcommunication. Accordingly, the control load in the base stations 2 athrough 2 c may be distributed to the mobile station 3. As a result, itis possible to improve the control efficiency for communication of thebase stations 2 a through 2 c with the mobile station 3.

Therefore, it is possible to increase the opportunity of communicationthat can be assigned to the mobile station 3 by the base stations 2 athrough 2 c. In other words, it is possible to increase the servicethroughput of the base stations 2 a through 2 c. In addition, theopportunity of communication for mobile station 3 in the communicationarea increases. The control efficiency of the base stations 2 a through2 c is also improved. As a result, it is possible to increase the datathroughput performed by the mobile stations 3 located in the end of thecommunication area. Accordingly, the range capable of achieving thetarget data throughput, that is, the cell coverage, can be enlarged.

Furthermore, the radio units 22 of the base stations 2 a through 2 c cantransmit the control signal including the traffic load in thecommunication area to the mobile station 3 through the downlink commoncontrol channel or downlink shared control channel. In this case, theradio unit 32 of the mobile stations 3 receives a control signalincluding the traffic load in the communication area, which istransmitted from the base stations 2 a through 2 c. Therefore, thecontrol unit 38 of the mobile station 3 may select the communicationarea for performing communication based on the traffic load included inthe control signal. In other words, the mobile station 3 can utilize thetraffic load transmitted from the base stations 2 a through 2 c,therefore, there is no need for the mobile station 3 itself to calculatethe traffic load. Therefore, the control load in the mobile station 3may be distributed to the base stations 2 a through 2 c. Meanwhile,control units 26 of the base stations 2 a through 2 c control tocommunicate with the mobile station 3 in the communication area selectedby the mobile station 3 based on traffic load. Accordingly, there is noneed for the base stations 2 a through 2 c to select the communicationarea for a plurality of mobile stations 3, and the control load of thebase stations 2 a through 2 c may be distributed to the mobile station3. Accordingly, it is possible to improve the control efficiency of thebase stations 2 a through 2 c, and to improve the efficiency with whichthe mobile station 3 selects the communication area as well. As aresult, it is possible to further improve data throughput.

When the control units 26 of the base stations 2 a through 2 c performproportional fairness scheduling, that is, the control unit 26 assigns achannel for the mobile station 3 with the largest ratio of the averagereception quality value over a short period to the average receptionquality value over a long period, the control unit 38 selects thecommunication area based on the ratio of the average reception qualityvalue over a short period to the average reception quality value over along period of the signal received by the radio unit 32. As a result,the control unit 38 may select the communication area for performingcommunication using the same criterion as the criterion for channelassignment.

Accordingly, the control unit 38 may select the communication area inwhich a channel is easily assigned, and the opportunity of communicationis easily assigned. As a result, it is possible to increase theopportunity on which the base stations 2 a through 2 c can assign thehigh speed packet channel for packet transmission to the mobile station3. Therefore, it is possible to increase the opportunity ofcommunication assigned to the mobile stations 3 by the base stations 2 athrough 2 c. As a result, it is possible to increase the data throughputperformed by the mobile stations 3 located at the end of thecommunication area. Accordingly, cell coverage may be enlarged.Furthermore, the mobile station 3 calculates the ratio of the averagereception quality value over a short period to the average receptionquality value over a long period, and then selects the communicationarea. Therefore, since it is possible to distribute the control load ofthe base stations 2 a through 2 c to the mobile station 3, and therebyimprove the control efficiency of the base stations 2 a through 2 c. Itis possible to further increase the data throughput performed by themobile station 3.

[Modified Example]

The present invention is not limited to the above mentioned embodiment,and various changes are possible. The case where a sector is used as thecommunication area unit, and the sector is selected as the communicationarea for performing communication is described. As shown in FIG. 12, thebase station 202 includes antennas 221 a and 221 b, a radio unit 222, abaseband processing unit 23, a buffer 24, a network interface unit 25,and a control unit 226. Since the baseband processing unit 23, buffer24, and network interface unit 25 are substantially the same as those inFIG. 2, a repetitive description is omitted by denoting the samereference numerals.

The radio unit 222 transmittes/receives data and control signal bysector. Therefore, the radio unit 222 includes a plurality oftransmission sectors 222 a and 222 b that function as a transmissionunit that transmits data and a control signal to a mobile station 3located in each sector and a reception unit that receives the data andcontrol signal from the mobile stations 3 located in each sector. Theantennas 221 a and 221 b transmit a packet and control signal input fromthe transmission sectors 222 a and 222 b to a mobile station 3. Theantennas 221 a and 221 b input the packet and control signal receivedfrom the mobile station 3 to transmission sectors 222 a and 222 b. Thetransmission sectors 222 a and 222 b transmit the packet and controlsignal input from the baseband processing unit 23 to the mobile station3 via the antennas 221 a and 221 b. The transmission sectors 222 a and222 b receive the packet and control signal transmitted from the mobilestation 3 via the antennas 221 a and 221 b, and input these to thebaseband processing unit 23.

The transmission sectors 222 a and 222 b cover respective sectors. InFIG. 12, only two transmission sectors 222 a and 222 b are shown,however, they are formed for the number of sectors that are obtainedwhen the cell covered by the base station 202 is divided. Thetransmission sectors 222 a and 222 b receive an instruction from thecontrol unit 226 to transmit a packet to the mobile station 3. Morespecifically, the transmission sectors 222 a and 222 b receive aninstruction from the control unit 226 to turn the packet transmissionfunction on. The transmission sectors 222 a and 222 b then transmit thepacket. Meanwhile, the transmission sectors 222 a and 222 b receive aninstruction from the control unit 226 not to transmit the packet to themobile stations 3. More specifically, the transmission sectors 222 a and222 b receive an instruction from the control unit 226 to turn thepacket transmission function off. The transmission sectors 222 a and 222b, accordingly, do not transmit a packet.

The transmission sectors 222 a and 222 b carry out substantially thesame function as the radio unit shown in FIG. 2 except for performingtransmission of the downlink control channel and high speed packetchannel by sector. The control unit 226 controls by sector. Morespecifically, the control unit 226 generates a control signal includingthe traffic load in the communication area by sector. The control unit226 also controls to communicate with the mobile station 3 in the sectorselected by the mobile station 3. The control unit 226 is substantiallythe same as the control unit 26 shown in FIG. 2 except for performingcontrol by sector.

More specifically, the control unit 226 obtains data indicating thecommunication area selected by the mobile stations 3 from the controlsignal transmitted from the mobile station 3. The control unit 226instructs the transmission sectors 222 a and 222 b that cover the sectorcorresponding to the obtained data to transmit the packet to the mobilestation 3. In other words, the control unit 226 instructs thetransmission sectors 222 a and 222 b to turn the packet transmissionfunction on.

Meanwhile, the control unit 226 instructs the transmission sectors 222 aand 222 b that cover the sector, which is not corresponding to theobtained data not to transmit a packet to the mobile station 3. In otherwords, the control unit 226 instructs the transmission sectors 222 a and222 b to turn the packet transmission function off. In this manner, thecontrol unit 226 controls to communicate with the mobile station 3 inthe communication area selected by sector.

Next, a communication method is described. In FIG. 13, proceduresperformed by components other than the transmission sectors 222 a and222 b, such as the control unit 226 and baseband unit 23 of the basestation 202 are shown as procedures of the base station 202. Inparticular, the procedures performed by the transmission sectors 222 aand 222 b are shown as the procedures in the transmission sectors 222 aand 222 b.

To begin with, as shown in FIG. 13, the transmission sectors 222 a and222 b transmit a control signal including the traffic load of therespective sectors covered by the transmission sectors 222 a and 222 bto the mobile station 3 through the downlink common control channel ordownlink shared control channel (S801). The respective transmissionsectors 222 a and 222 b also transmit the pilot signal to the mobilestation 3 through the common pilot channel (S802).

The radio unit 32 of the mobile station 3 receives the common pilotchannel from the respective transmission sectors 222 a and 222 b. Theradio unit 32 measures the received power of the received common pilotchannel, and inputs the result to the control unit 38. The control unit38 finds the reception quality, such as the received power or signal tointerference power ratio, by using the pilot signal transmitted throughthe common pilot channel from the respective transmission sectors 222 aand 222 b (S803). The control unit 38 selects the transmission sectors222 a and 222 b that cover the sector with reception quality within thethreshold value, from the transmission sectors 222 a and 222 b thatcover the sector that has the most preferable reception quality as thetransmission sector candidates to cover the sector for performingcommunication (S804). The control unit 38 selects the sector with theleast traffic load included in the control signal transmitted in step(S801) from the selected transmission sector candidates. In this case,since the communication area unit is a sector, the mobile station 3selects the transmission sector 222 a or 222 b that covers the selectedsector (S805).

The control unit 38 of the mobile station 3 generates the control signalincluding the sector ID of the sector covered by the selectedtransmission sector 222 a or 222 b. The radio unit 32 multiplexes thecontrol signal in the uplink channel, and transmits the control signalto the base station 202 (S806). In the base station 202, the radio unit222 receives a control signal including the sector ID selected by themobile station 3, which is transmitted from the mobile station 3 throughthe uplink channel. The baseband processing unit 23 then demodulatesdata (S807). The control unit 226 of the base station 202 instructs thetransmission sector 222 a or 222 b which covers the sector correspondingto the sector ID included in the control signal to turn the packettransmission function on. The control unit 226 instructs thetransmission sector 222 a or 222 b which covers the sector notcorresponding to the sector ID included in the control signal to turnthe packet transmission function off (S808).

The respective transmission sectors 222 a and 222 b determine whetherthe instruction to transmit the packet is received from the control unit226 (S809). The transmission sector 222 a or 222 b transmits the packet(S810) when receiving the instruction to transmit the packet in step(S809). Meanwhile, the transmission sector 222 a or 222 b does nottransmit the packet (S811) when receiving the instruction not totransmit the packet in step (S809).

In this manner, even when the communication area unit is a sector, themobile station 3 may select the communication area for which opportunityof communication is easily assigned, as the communication area forperforming communication. Accordingly, the base station 202 can increaseservice throughput. The opportunity for a mobile station 3 to performcommunication in the communication area increases. As a result, it ispossible to increase the data throughput performed by the mobile station3 located in the end of the communication area.

Note that in the communication method shown in FIG. 13 the sector isselected based on the traffic load in the sector included in the controlsignal and the reception quality in the sector, however, the sector maybe selected by estimating the traffic load in the sector, or based onthe ratio of the average reception quality value over a short period tothe average reception quality value over a long period.

In FIG. 9 through FIG. 11, the mobile stations 3 select thecommunication area when the base stations 2 a through 2 c performproportional fairness scheduling as the channel assignment method,however, the base stations 2 a through 2 c may select the communicationarea. In this case, the control unit 38 of the mobile stations 3generates a control signal including reception quality informationrelated to the reception quality, such as the received power or signalto interference power ratio found based on the received pilot signal.The control unit 38 instructs the radio unit 32 to transmit the controlsignal to the base stations 2 a through 2 c. The radio unit 32 thentransmits the control signal including the reception quality informationto the base stations 2 a through 2 c. The radio unit 22 of the basestations 2 a through 2 c receives the control signal including thereception quality information. The control unit 26 obtains the controlsignal from the radio unit 22 via the baseband processing unit 23.

The control unit 26 obtains the reception quality information includedin the control signal. The control unit 26 then calculates the averagereception quality value over a short period to the average receptionquality value over a long period of the received signal received by themobile stations 3, using the reception quality information. The controlunit 26 then functions as a calculation unit to calculate the ratiothereof. The control unit 26 functions as a selection unit that selectsthe communication area with the largest ratio as the communication areawhere the mobile station 3 performs communication, based on thecalculated ratio. The control unit 26 of the base stations 2 a through 2c assigns the high speed packet channel for packet transmission to themobile station 3 when the base station that covers the selectedcommunication area is that base station itself. The radio unit 22 thentransmits the packet. Meanwhile, the control units 26 of the other basestations 2 a through 2 c perform no operations when the base stationthat covers the selected communication area is not the other basestation.

Furthermore, the communication area may be selected in consideration ofthe reception quality of the signal received by the mobile station 3even in the case of a communication method that selects thecommunication area through estimation of the traffic load by the mobilestation 3 as shown in FIG. 6. For example, the mobile station 3 mayselect the communication area with a small estimated traffic load fromthe communication areas where there is some degree of preferablereception quality. The control unit 3 may also select the communicationarea having preferable reception quality from communication areas wherethe estimated traffic load is somewhat little.

In the communication methods shown in FIG. 7, FIG. 8, and FIG. 12, themobile station 3 selects the communication area candidates inconsideration of the reception quality first, and selects thecommunication area with little traffic load from those candidates,however, the communication area candidates may be selected inconsideration of the traffic load first, and then communication areawith preferable reception quality may be selected from those candidates.Similarly, in the communication method shown in FIG. 11, the mobilestation 3 selects communication area candidates for performingcommunication in consideration of the traffic load first, and thenselects the communication area with large ratio from those candidates,however, the communication area candidates with a large ratios may beselected first, and the communication area with little traffic load maybe then selected from those candidates.

1. A mobile station comprising: a reception unit configured to receive asignal transmitted from a base station; a selection unit configured toselect a communication area to perform communication from a plurality ofcommunication areas based on traffic load in the plurality ofcommunication areas where the reception unit can receive the signal; anda notification unit configured to notify the base station of thecommunication area selected by the selection unit.
 2. The mobile stationof claim 1, wherein the reception unit receives a control signalincluding the traffic load in the communication area transmitted fromthe base station through a control channel; wherein the selection unitselects the communication area based on the traffic load included in thecontrol signal.
 3. The mobile station of claim 1, further comprising anestimation unit configured to estimate the traffic load in the pluralityof communication areas based on received power of the signal received bythe reception unit, wherein the selection unit selects the communicationarea based on the traffic load estimated by the estimation unit. 4.(canceled)
 5. A base station comprising: a transmission unit configuredto transmit a control signal including traffic load in a communicationarea to a mobile station through a control channel; and a control unitconfigured to control to communicate with the mobile station in acommunication area selected by the mobile station.
 6. (Canceled).
 7. Acommunication system comprising: a mobile station that includes areception unit configured to receive a signal transmitted from a basestation, a selection unit configured to select a communication area toperform communication from a plurality of communication areas based ontraffic load in the plurality of communication areas where the receptionunit can receive the signal, and a notification unit configured tonotify the base station of the communication area selected by theselection unit; and the base station that includes a control unitconfigured to control to communicate with the mobile station in thecommunication area notified from the notification unit.
 8. Acommunication method, comprising selecting by a mobile station, acommunication area where the mobile station performs communication, froma plurality of communication areas, based on traffic load in theplurality of communication areas capable of receiving a signaltransmitted from a base station; notifying the base station of theselected communication area; and controlling by the base station, tocommunicate with the mobile station in the communication area notifiedfrom the mobile station.
 9. The communication method of claim 8, whereinthe base station transmits a control signal including the traffic loadin the communication area to the mobile station through a controlchannel; and the mobile station receives the control signal and selectsthe communication area based on the traffic load included in the controlsignal.
 10. The communication method of claim 8, wherein the mobilestation estimates the traffic load in the plurality of communicationareas based on received power of a received signal; and selects thecommunication area based on the estimated traffic load.
 11. Thecommunication method of claim 8, wherein the mobile station selects thecommunication area based on reception quality of a received signal andthe traffic load in the plurality of communication areas.
 12. (canceled)